Most micromachined devices can be split into two major classes: those formed by surface micromachining methods, and those formed by bulk micromachining methods. Surface micromachining methods are based on patterning films deposited on a substrate; whereas bulk micromachining methods are based on sculpting a bulk substrate using etchants.
Surface micromachining processes generally use deposited low-stress polycrystalline silicon (also termed polysilicon) layers to form elements of a micromechanical structure, with sacrificial layers (e.g. comprising silicon dioxide or silicate glass) separating the polysilicon layers from each other and from the substrate. Since the layers are patterned by photolithography, they can be very accurately defined and have essentially arbitrary lateral shapes. After defining the micromechanical structure in the polysilicon layers, the sacrificial layers are removed with a selective etchant comprising hydrofluoric acid (HF) to release the micromechanical structure for operation.
Although surface micromachining has proven useful for fabricating many types of micromechanical devices including accelerometers, motors, gear trains and moveable mirrors, such surface-micromachined devices are limited in the vertical dimension (i.e. the out-of-plane dimension which is generally referred to as the Z dimension) to an overall thickness of about 6 .mu.m for the polysilicon layers and about 3 .mu.m for the sacrificial layers. This limitation in overall thickness of surface-micromachined devices is primarily due to problems of stress in the layers, with the low deposition rate for polysilicon also being a limiting factor. Accordingly, surface-micromachined devices are essentially two dimensional in structure with a low stiffness in the Z dimension; and this limits, for example, the force possible from actuators (e.g. electrostatic comb actuators) and linkages, and the mass and sensitivity of inertial sensors (i.e. accelerometers).
The low stiffness in the Z dimension can also cause problems due to stiction whereby, after release, elements of the surface-micromachined device can reattach or stick to the underlying substrate due to a variety of causes including static electricity. Fabrication of surface-micromachined devices, therefore, can require additional, and often elaborate, process steps to prevent or overcome stiction.
Another problem with surface-micromachined devices is that these devices generally require a long-duration high-temperature annealing step to relieve stress in the polysilicon layers. This high-temperature annealing step greatly complicates the problem of integrating surface-micromachined devices with electronic circuitry (e.g. CMOS or bipolar circuitry) since the annealing temperature (typically 700-1300.degree.C.) exceeds the melting point of aluminum that is commonly used as the interconnect metallization for integrated circuits.
An additional problem with the use of surface micromachining is the need for an aggressive HF-based etchant for release of the surface-micromachined devices. The etch release step can take many hours during which time other elements of the device such as electronic circuitry must be overcoated to protect these elements from attack by the HF-based etchant.
Finally, surface-micromachined devices fabricated with polysilicon can have properties inferior to the properties of devices fabricated with a crystalline structure. For example, the granularity of polysilicon results in a lower mechanical strength and inferior electronic properties than those of single crystal silicon. Additionally, the light reflecting properties of deposited polysilicon are inferior to those of crystalline silicon.
In bulk micromachining, a silicon substrate is etched and sculpted with a wet etchant to form relatively massive micromechanical structures such as accelerometers, or fluid valving and delivery systems. The attraction of bulk micromachining is that it can produce parts which have a high stiffness in the Z dimension, and relatively large masses for inertial sensors. The present disadvantage of bulk micromachining is that there are only a limited number of possible shapes that can be fabricated since bulk micromachining processes are typically highly dependent on the crystal orientation of the substrate. Additionally, bulk micromachining generally produces shapes that have sidewalls that are not orthogonal to a top surface of the substrate. The shape of elements of a bulk-micromachined device can also be controlled to a limited extent by the use of a high dose of implanted boron which acts as an etch stop. However, in general, the type, shape and size of micromechanical structures that can be fabricated by bulk micromachining is severely limited. In particular, the formation of fine features of arbitrary shape on a micron-size scale has not been possible with bulk micromachining as practiced heretofore.
What is needed is a micromachining method that combines the best features of both surface micromachining and bulk micromachining. Such a method should allow the formation of lateral features of arbitrary size with micron-scale resolution, while at the same time allowing the rapid and accurate removal of underlying material to a predetermined stopping point. According to the present invention, such a method is possible by the substitution of a {111}-oriented silicon substrate for the commonly used {100}-oriented silicon substrates and by the use of anisotropic dry etching in combination with anisotropic wet etching.
An advantage of the method of the present invention, based on the use of a {111}-oriented silicon substrate, is that {111} planes of silicon which are parallel to and form the major surfaces of the substrate can be used to control an anisotropic wet etching step that is used to rapidly and accurately form one or more planar surfaces of a micromechanical device (e.g. to form a plate or beam of the device).
A further advantage of the method of the present invention is that the wet etching step can be used to partially or completely undercut one or more elements of the micromechanical device, forming a lower surface of the device which is co-planar with an upper surface of the device and stopping at a predetermined location as determined by an angled {111}-silicon plane or by a fabricated etch-stop.
Another advantage is that the method of the present invention can be used to form a micromechanical structure having a layer thickness in the range of a few microns to hundreds of microns to provide a high stiffness in the vertical (Z) dimension.
Yet another advantage is that the method of the present invention can be used to form a micromechanical structure that is spaced away from the substrate by sufficient distance to substantially reduce a parasitic capacitance between the structure and the substrate.
Still another advantage is that the method of the present invention can be used to form a micromechanical device having lateral dimensions in the range of about one micron up to thousands of microns.
Another advantage of the method of the present invention is that micromechanical structures can be formed from single-crystal silicon, thereby providing improved mechanical strength, low stress improved electrical characteristics and/or improved light reflection characteristics as compared to structures formed from polysilicon.
Yet another advantage of the method of the present invention is that all process steps of the method can be carried out at temperatures below the melting point of aluminum, thereby allowing the fabrication of integrated circuitry on the substrate before or simultaneously with the process steps used to fabricate the micromechanical device.
These and other advantages of the method of the present invention will become evident to those skilled in the art.